Measurement of echo effects in transmission systems



Jan. 14, 1930. D. THIERBACH 1,743,403

' MEASUREMENT OF ECHO EFFECTS IN TRANSMISSION SYSTEMS Filed March 11, 192"! Va I Z Ve Patented Jan. 14,1930

UNITED STATES PATENT OFFICE DIETWALD THIEBB ACH, OI CHABLOTTENBURG, BERLIN, I GERMANY, ASSIGNOR T 8 & HALSKE AKTIENGESELLSCHAFT, OF SIEMENSSTADT, NEAR BERLIN,

GERMANY, A COMPANY OF GERMANY MEASUREMENT OI ECHO EFFECTS IN TRANSMISSION SYSTEMS Application filed March 11, 1927, Serial No. 174,589, and in Germany April 87, 1988.

The invention relates to a method which enables the echo-eifects occurring in long tele- 20116 connections to be measured. 7 As is own, echoes may occur in a telephone communication system, for instance, in a four wire transmission system, because a part of the incoming speech currents passes into the return line owing to inaccurate line simulation at the branching points and thus the 10 speech currents become again audible at the calling station. I

According to this invention the measure-- ment of echo-eflects consists in comparing the echo currents or echo voltages returning to the calling station with the outgoing original currents or voltages. The invention is based on the fact that a line in which an echoefiect occurs may be replaced with respect to the calling station by an active bi-polar device, for instance, by an impedance of the magnitude of the impedance of the outgoing line in series with the E. M. F. of the echo.

Fig. 1 shows, by way of example, the calling station, represented by a microphone M in series with the microphone battery B, which produces an initial voltage Va at the input end of the line to be tested. With res ect to the voltages and currents present at t e input of the line, the connection is re- 39 placed by an impedance Z in series with the E. M. F. of the echo Ve. Here Ve is always proportional to the impressed voltage Va; approximately Ve=k Va, where k can be conveniently called the echo-factor. When the echo disappears, for example, when one of the return wires of a four-wire connection is broken, the input impedance of the line Z =Z, where as when the return line is efiective 40 Z Thus the echo-factor k can be determined, for example, by measuring the impedance Z of the outgoing line when the return line is broken, and the impedance Z, when it "is efi'ective. From these two measurements the echo-factor may be determined by the equation It is advisable to call the natural logarithm of I: the echo-attentu'ation in conformity with the nomenclature which is usual in the telephone art. This echo attenuation can be determined according to the present invention for example withthe aid of a device which consists essentially of an impedance measurin bridge. From the measured values Z an Z, the echo-attenuation is given by Fig. 2 shows as an example a further measurin device for determinin' the echo; this also eing based upon the 1. ct that a line with an echo can be replaced by a circuit in which an orginating source and an echo source are present.

The arrangement consists substantially of a bridge comprising resistances R R R R The voltage of source S tapped ofi' at the brid e arm R acts through a resistance R, whic should be approximately e ual toZ, and is impressed on the in ut o the line which is to be investigated. 1 is a resistance which is small with respect to Z or R and the ual to the voltage Va passinginto the line.

I however, there is an echo voltage Va active, the telephone T gives a response which is proportional to the echo voltage.

.aline'to Preferably the resistance of the telephone or other indicating instrument is made great with respect to the resistance R. The v0 e impressed on the telephone T is then for all practical purposes given by V Ve/2. Then the'indication given at T, is compared with the indication arising at T For this purpose R is preferably designed as a potentiometer.

If, for example, the 'mth portion of the voltage Va prevailing at B is tapped off for T then the voltage at T is given by V Va. If the two telephones give the same sound,

then V,=V, or Ve= i Va. Fromthis the echo-factor or the echo-attenuation can be calculated from the known dividing ratio m. It is within the scope of the invention to emgloyany other suitable indicating means to etermine when the voltages across T and T are equal.

What is. claimed is:

1. In an echo intensity measuring circuit, abridge circuit therefor, a source of potential sup lied through said bridge circuit to tested, and means included in said bridge circuit for determinin the ratio of the potential impressed on the 'ne to the echo potential. I

2. In an echo intensity measuring circuit, a bridge circuit for supplying a potential to the line to be tested, said bridge circuit including a potentiometer for comparing the echo potentials to the impressed potentials,

said potentiometer determining the ratio of the; echoprotentials to the impressed potentia s. .7

3. In an echo intensity measuring circuit, a Wheatstone bridge arrangement including a potentiometer, a source of alternating current to be impressed on a test line through said bridge, an indicating device across said bridge balanced with respect to said source, said indicating device being responsive to otentials in said test line, another indicating device whose response is to be compared with Q that of said first indicating device, said other indicating device being responsive to otentials arising at said source. andderive from said potentiometerin ,which currents from said source are not balanced out.

In witness whereof, I'hereunto subscribe my name this 21st day of February, A. D.

DIETWALD TH IERBACH. 

